The present invention relates to battery chargers and, more particularly, to battery chargers that are physically compact, inexpensive to fabricate, and well suited for charging batteries used in battery-powered consumer products.
Various types of circuits have been developed for charging batteries, typically nickel--cadmium (Ni--Cd) batteries, used in battery-powered consumer products such as small appliances and portable, hand-held power tools. In general, the charging circuit must be relatively simple in design and inexpensive to fabricate since complexity and high cost are inconsistent with the need to provide low-cost products to the consumer market. Where the battery charger circuit is integrated with the product, for example, a hand-held, battery-powered electric drill having a built-in charger circuit, it is important that the circuit be physically compact and lightweight to allow maximum design flexibility in configuring the final product.
In designing a battery charger circuit, consideration must be given to the rate of charging and the risk of overcharging, since repeated overcharging shortens the useful life of the battery. In the past, low-current charging circuits, known as `trickle` chargers, have been used to charge the battery by supplying a relatively low current over a long period of time, e.g., 8-16 hours. The current level is usually low enough so that overcharging will not occur, even if the charging current is continued for an indefinite period. While low-current battery chargers avoid the problem of overcharging, their long charging cycles are not acceptable from the consumer standpoint. While higher current battery chargers, viz., the `fast` chargers, can bring a battery to a full charge in an hour or so, the risk of overcharging the battery is dramatically increased in comparison to low-current chargers. As can be appreciated, a optimal battery charger would effect charging in a relatively short time, as in the case of the fast charger, and would minimize the probability of overcharging the battery, as in the case of the trickle charger.
Since battery-powered products operate at relatively low voltages, charger circuits have included step-down transformers that step the power source voltage, typically 120 VAC, down to a level consistent with requirements of the battery, e.g., 12 volts, and, additionally, `isolate` the low voltage portions of the charger circuit from the higher voltage power source. Unfortunately, transformers are relatively expensive, and, where the charger circuit is to be integrated into the product housing, represent a component that is physically larger and heavier than the other circuit components typically used in charging circuits. Thus, in a battery charging circuit designed for use in consumer products, especially where the charger circuit must be integrated into the product housing, it is desirable that the transformer be eliminated for cost and design considerations.